1. Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic automotive type transmission and more specifically to a lock-up clutch control arrangement for use with such a system.
2. Description of the Prior Art
JP-A-61-206868 discloses a lock-up control arrangement wherein engine load as indicated by engine throttle valve opening degree and vehicle speed are used to determine when a lock-up clutch should be fully released and induce a so-called open converter mode of operation, when the clutch should be partially engaged to induce what is referred to as slip lock-up and when the clutch should be strongly engaged to produce what shall be referred to as full lock-up.
However, with this type of arrangement, a drawback has been encountered in that during, slip or full lock-up modes of operation, if the accelerator pedal is subject to a sudden change in depression degree, the instant lock-up status is maintained until predetermined engine load/vehicle speed requirements are satisfied and the driver is unable to obtain the acceleration characteristics which would be expected for the given amount of pedal depression in response to the pedal depression.
That is to say, due to the load on the engine, when there is a delay between the actual change in throttle position (induced by the accelerator pedal depression) and the actual change in the engine speed.
In the case in which the solenoid valve which controls the lock-up is subject to electronic control, during slip or full lock-up modes of operation, it has been proposed that the rate at which the throttle opening changes be predetermined and if this rate exceeds a predetermined value, the lock-up clutch is conditioned to assume a fully released condition.
However, with this type of proposal, if a fail safe step is included, the control program is rendered undesirably complex and increases the production costs.